Antenna tuning circuit



June 17, 1941. .D. E. FOSTER ANTENNA TUNING CIRCUIT Filed Dec. 16, 19376 M mum R r m M 5 11 m 2232 H 25% 7 m M w, A W A 5m 1 r. F 8 m H a 4 f Lm mu E w f 1 w a 0 DA" 1/ W \H" 0 a m VLV l INVENTOR m w m E/Dm PatentedJune 17, 1941 TES P s era ANTENNA TUNING CIRCUIT Dudley E. Foster, SouthOrange, N. J assignor to Radio Corporation of America, a corporation ofDelaware My present invention relates to signal receiving systems, andmore particularly to a radio receiving system utilizing a tunable signalcollector.

It is well known that a high signal to noise ratio at the signalcollector of a radio receiving system is beneficial. For this reasonradio receivers, particularly those adapted to receive signals in thebroadcast range, are provided with signal collector circuits of highgain. For example, a high gain signal collector, or antenna circuit,usually employs an inductance element of large magnitude. However, asthe receiving system employed with such a high gain signal collector isvaried in frequency through the tuning range, the collector gaindecreases by virtue of the fact that the collector is naturally resonantto a given frequency of the receiver tuning range. While it would befeasible to provide a variable capacity to tune the signal collectorover the operating signal range of the receiver, the variable capacitybeing ganged with the receiver tuning control condensers, yet such adevice would not be practical because a variable capacity cannot be usedin a practical manner to tune the signal collector circuit. On the otherhand, it is equally undesirable to employ a variable inductor in theantenna circuit, since it would be necessary to gang the latter with thevariable tuning condensers of the receiver.

Accordingly, it may be stated that it is one of the main objects of mypresent invention to provide a signal collector circuit for a tunableradio receiver, wherein the collector circuit is constructed I to have ahigh gain, and an electronic device being electrically associated withthe signal collector circuit so that the latter may be conjointly tunedwith the receiver over the tuning range thereby to preserve a highsignal to noise ratio at the collector circuit.

Another important object of my invention may be stated to reside in theprovision of a radio receiver provided with a tuning instrumentality,and the receiver being equipped with a signal collector circuit having ahigh inductance value thereby to provide a high signal to noise ratio atthe low frequency end of the receiver tuning range, and means beingemployed to tune the collector circuit conjointly with the receiver overthe tuning range, and without physically changing the inductance in thecollector circuit, thereby to preserve said ratio over the tuning range.

Another object of my invention may be stated to provide a signalcollector circuit adapted to be used, in general, for radio receiverswhich are tunable over a wide range of signal frequency, the collectorcircuit being of the type having a reactive element of high inductivemagnitude thereby to provide a high signal gain at the low a frequencyend of the said signal range, and an electronic device beingelectrically associated with the collector circuit reactive element sothat the electronic device may be adjusted to vary the inductivemagnitude of said reactive element in a sense to adjust the collectorcircuit frequency over the wide signal range.

Still other objects of my invention are to improve generally thesimplicity and efiiciency of radio receivers, and more particularly toprovide more efficient signal collector circuits for radio receivers,and which collector circuits are not only reliable in operation, but areeconomically manufactured and assembled in radio receivers.

The novel'features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood'by referenceto the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention'maybe carried into effect. 1

In the drawing: 1

Fig. 1 shows a circuit arrangement embodying one form of my invention,and

Fig. 2 illustrates another embodiment of invention.

Referring now to the accompanying drawing,

the

.wherein like reference characters in the two figures designate similarcircuit elements, there is shown in Fig. 1 a signal collector circuit ofthe grounded antenna type. The numeral l designates the grounded antennacircuit, and the antenna circuit includes the coil 2. The highalternating potential end of coil 2 is connected, as by signal frequencycoupling condenser 3, to the input electrode of the first radiofrequency amplifier 4. The amplified radio frequency signals areimpressed upon the tunable signal selector circuit 5 which is adapted tobe connected to the input electrodes of the following signaltransmission tube. The selector circuit 5 includes a variable condenser6 which is adapted to vary the tuning of selector circuit 5 over adesired signal frequency range. It is to be understood that the numeral6 denotes the usual tuning device employed for broadcast receivers; thatis, receivers operating at a signal range of 550 to 1500 kc.

If the receiver is of the superheterodyne type, then the selectorcircuit 5 may feed into a first detector, or an additional radiofrequency amplifier stage; the stages following the first detector aretoo well known to be recited. The dotted line connected to condenser 6has been designated as the Tuner, and it will be understood that thisinstrumentality is adapted to vary the rotors of a gang of variablecondensers usually employed in a broadcast receiver. If the receiver isof the tuned radio frequency amplifier type, then numeral 6 representsthe gang of variable condensers employed in the cascaded signal selectorcircuits. The magnitude of coil 2 should be chosen so that it resonateswith the inherent capacity of antenna I to a frequency at the lowfrequency end of the receiver tuning range, the tube I being biased tocut-off. For example, in the broadcast band of 550 to 1500 kc., theantenna circuit including coil 2 will be series resonant to a frequencyof 550 kc.

As the tuning device of the receiver is varied toward the high frequencyend of the tuning range, it is necessary to adjust the tuning of thesignal collector in synchronism with the receiver tuning device. Asstated before, it is not practical to do this by utilizing a variablecapacity in the antenna circuit; and it is not desirable to physicallyvary the coil 2. According to my invention, however, the collectorcircuit is varied in frequency by the relatively simple and eflicientdevice shown in Fig. 1. There is utilized an electron discharge tube 1,say of the pentode type, which has a cathode 8, a control grid 9 and aplate Ill. The usual positive screen grid and a suppressor grid areemployed between the plate II! and control grid 9. The control grid 9 isconnected to the junction of resistor II and condenser I2, the latterelements being connected in series across the coil 2. The connectionbetween control grid 9 and the junction of resistor II and condenser I2is made through a condenser I3 which has a low impedance to currents ofsignal frequency. The cathode 8 is connected to ground through aresistor-condenser network I 4 which functions to provide a fixed biasfor the control grid 9. It is desirable to choose the bias such asnormally to cut off the flow of plate current in tube I, when the tuningdevice of the receiver is adjusted to the low frequency end of thetuning range.

The plate I9 is connected to a source of proper positive potential, asthe power supply bleeder resistor of the receiving system, through acoil I5. The plate Ii is,'also, connected to the high potential end ofcoil 2 through a condenser I6. There is provided an adjustablepotentiometer for varying the bias of the control grid 9, and thepotentiometer comprises a bleeder resistor I! having an intermediatepoint thereon at ground potential. An adjustable tap IB is arranged toslide along theresistor I'I between the grounded point thereon and thepositive potential terminal thereof. The control grid 9 is connected tothe tap I8 through a connection including resistor I9.

Of course, the bleeder resistor I "i may be a portion of the commonpotential supply circuit of the receiving system. The broken line 29denotes a mechanical coupling between the adjustable tap I8 and thetuning device of the receiving system. This mechanical coupling 20' maybe of any desired construction, it being only necessary to point outthat the coupling should be such that as the rotors of the gang ofvariable condensers are adjusted to tune the receiver selector circuitsfrom the low frequency end of the tuning range toward the high frequencyend thereof, then the tap I8 will be adjusted from the grounded point onbleeder resistor II toward the positive potential terminal.

When the receiver is tuned to the low frequency end of the tuning range,then it is to be understood the tap I8 will be at the grounded point onthe bleeder resistor II. In that case a plate current cut-off bias willbe impressed on the control grid 9 by virtue of the voltage drop acrossthe bias network I4. This means that the tube I will be inoperative, andwill produce no effect as far as coil 2 is concerned. However, as thetuning device is adjusted to increase the frequency of the receiverselector circuits, the tap I8 will be adjusted in a positive potentialsense and, hence, increase the gain of tube 1. An increase in the gainof tube I will result in the reduction of the inductive magnitude ofcoil 2, and the decrease in inductive magnitude of coil 2 will be at arate such that the antenna circuit frequency will increase insynchronism with the frequency of the receiver selector circuits. Thisproper and synchronous change in the inductive magnitude of coil 2 issecured by proper adjustment of tap I8 along the bleeder resistor I1.

The tube 1, and its input and output connections to coil 2, functions asa quadrature tube. That is, the plate current flowing through coil 2from tube 1 is in quadrature relation with the voltage across that coilby reason of the voltage impressed on the control grid 9 and derivedfrom the junction of resistor II and condenser I2. As the gain of tube 1increases, the quadrature current increases, and this, in turn,increases the inductive effect of tube I which is in shunt across coil 2thereby decreasing the inductive magnitude of the coil. Condenser I6 andcoil I5 can have their magnitudes chosen so that they are seriesresonant at some frequency producing a large impedance change at thatfrequency. In the form of circuit shown in Fig. l, the plate current oftube I actually flows through coil I5, but since the latter coilis inshunt relation with coil 2, the increasein quadrature current flowthrough coil I5 actually results in a decrease in the inductivemagnitude of coil 2. When the plate potential for tube 1 is suppliedthrough coil I5, then a signal frequency by-pass condenser 2| isconnected between the grounded end of coil 2 and the low potential endof coil I 5. The quadrature tube 1 is limited in the inductive effect itcan produce. It can act like a high inductive, but it is limited by itsmutual conductance to the lower limit of inductance effect it canproduce in shuntwith a coil.

By adding the condenser 39 in series with coil 2 in the antenna circuitI, as shown in Fig. 2, it is possible to use a much larger inductancemagnitude for coil 2 than in the case of Fig. 1. Hence, a greatervariation of signal collector frequency is possible, and the tube 1 maybe chosen to have a relatively lower mutual conductance than in the caseof the circuit shown in Fig. l. The arrangement in Fig, 2 differs fromthat shown in Fig. 1 in that the plate In is connected directly to thejunction of condenser 30 and coil 2; the low potential end of coil 2 maybe connected to a point B of positive potential. The cathode 8 of. thequadrature tube 1 may be connected to ground, and instead of using aspe-- cial self-biasing network I4, the tap I8 may be adjusted to thenegative side of the bleeder resistor H and to a point thereon such thatcutoff bias is applied to control grid 9. In other words, by sliding tapit from the negative sec- -tion'of bleeder resistor ll toward thepositive section thereof, the gain of tube '1 may be controlled. Theoperation of the circuit shown in Fig. 2 is similar to that of Fig. i.There is impressed on the control grid a signal potential which is inquadrature relation with the plate current flowing from tube 1 throughcoil 2.

Let us consider Fig. 2, and assume that condenser 30 is so large that itexercises no tuning function; say it has a value of 0.01 rnrnf. Let usfurther assume that tube i has a maximum mutual conductance of 1500micrornhos. With tube 1 cut 01?, coil 2 should be resonant with thecapacity of antenna 5 to the lowest desired tuned frequency; say 550 kc.Assuming that the antenna has a typical value of capacity of 150 mmf.,coil 2 must have an inductance value of 550 mh. The radio frequencyresistance of such a coil is usually of the order of ohms at 550 kc.,and increases approximately proportional to frequency. The antennaresistance may likewise be of the order of 10 ohms; in which case theresonant step-up of the antenna circuit will have a value of 75, whereaswith ordinary circuits a value of antenna circuit step-up of isconsidered good. Now, as has been shown in the technical literature, theeffective inductance value produced by tube 1 in the connection shown isgiven by the expression .RC/Gm, where R is the value of resistor I i, Cis the value of capacity 52, and Gm is the mutual conductance of tube 1.

For maximum tuning range the minimum value of efiective inductanceproduced by tube i should be as low as possible, and, as the expressionabove indicates, this means a low value of R and C and a high value ofGm. However, there is a practical lower limit to the value of R, sincethe lower R is, the greater will be its effect on the power factor ofcoil 2 and the less will the gain value circuit. The effect of a givenvalue of R will be greater if coil 2 and antenna l have low losses (lowresistance) than if they have high resistance. Furthermore, 'a minimumto the value of C is set in practice by the input capacity of tube 5 andthe distributed capacity of the associated circuit wiring. This usuallylimits the minimum value of C to about 10 mmf. The limit to Gm is set byavailable tubes, and is usually between 1000 and 2000 microinhosmaximum. This means that for a frequency range of tuning of 3 to 1, R.would have to be approximately 10,000 ohms, since the minimum value ofinductance due to coil 2 and effective inductance of tube 7 in parallelmust be about 56 rnh. If this value were achieved with Gm of 1500micrornhos, the receiver would tune from 550 to 1750 kc. However, therequired value of R. to accomplish this we have seen to be 10,000 ohms;but this value results in so unfavorable a power factor for the circuitthat the gain would be only 3 or 4, in place of the 75 noted above. Itis obvious, therefore, the R cannot be as small as 10,000 ohms, if highgain is to be realized in the antenna stage.

If now, the value of antenna capacity were halved, the value of coil 2necessary to tune to the lowest desired frequency would be doubled, andthe value of equivalent inductance required to be produced by tube l totune to the highest frequency would likewise be doubled, allowing theuse of a larger resistor R (resistor l i). The same result, as far asthe circuit is concerned, can beobtained by inserting a capacity 30, asby using a low capacity antenna. For example, if the antenna capacity of150 mmf. considered previously be employed, and condenser 30 has thevalue of mrnf., the antenna circuit will have a capacity of 37.5 mmf.and coil 2 will have a value of 22.40 mh. Tube 1 will then only berequired to produce a minimum of 315 mh, to tune from 550 to 1500 kc.Using the same value of C and Gm as before, R may be 47,000 ohms andobtain this inductance value. The gain obtained with this value of coil2 and R would be approximately 8. A still further reduction of the sizeof condenser 30 would permit the use of a larger resistor II withconsequent higher gain.

If, now, we consider Fig. 1, there are two resonant circuits capable ofadjustment, the capacity of antenna 1 and the inductance of coil 2 whichwill be chosen to resonate at the lowest desired frequency as before,and coil l5 and condenser i0. Coil may be large, say of the order of 5millihenries, in which case it has been shown that a given minimum valueof effective inductance due to tube I produces a greater tuning effectthan if coil l5 were small. But this high value of inductive reactancedue to coil i5 and tube 1 will not usually be sufficiently low to tuneto the highest desired frequency, unless condenser l0 be added in seriestherewith. The reactance of condenser 56 being opposite in sign to thatof coil l5 and tube 1, it

' will cause a lower total net effective reactance across coil 2, andthus cause variations of inductance due to changing the grid bias ontube l to produce a greater tuning eifect.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. An input system for a radio receiver consisting of an inductanceconnected between a signal collector and ground, .a series combinationof a capacity and a second inductance effectively connected in shuntwith the first inductance, a tube having its output connected acrosssaid second inductance and having its input circuit connected to aquadrature circuit connected across said second inductance, and meansfor varying the direct current grid bias on the tube whereby theresonant frequency of the said entire input system is caused to vary inresponse to the variations of the grid bias potential.

2. A receiving system having in combination a selector circuit tunableover a range of signal frequencies, an antenna circuit coupled to theselector circuit and having a high inductance coil which resonates withthe inherent capacity of the antenna circuit at the low frequency end ofthe above range of frequencies, a thermionic impedance effectively inshunt to said antenna circuit coil for controlling the effectiveimpedance of saidcoil, means for varying said thermionic impedance, andmeans for adjusting the tuning of the selector circuit conjointly withadjustment of said impedance varying means.

3. A receiving system having in combination a selector circuit tunableover a range of signal frequencies, an antenna circuit coupled to thetuning of the selector circuit conjointly with selector circuit andhaving a high inductance adjustment of said impedance varying means.coil which resonates with the inherent capacity 4. A receiving systemaccording to claim 3 of the antenna circuit at the low frequency endwherein the impedance varying means comprises of the above range offrequencies, an electron 5 the control grid of the electron dischargetube,? discharge tube having its space discharge path and the tube isbiased to substantially plate effectively in shunt to said antennacircuit coil, current cut-ofi with adjustment of the selector means forvarying the impedance of said discircuit to the low frequency end of thefrequency charge path and in consequence the efiective imrange.

pedance of said coil, and means for adjusting the 10 DUDLEY E. FOSTER.

